So having a cold wallet for a casino is recommended because that can prevent hacking that can happen anytime.
Cold wallets has nothing to do for security of the gambling website to prevent the hacking.
If you mean it's used to minimize the damage of lost funds taken when a hack happens, then it's reasonable.
In any way, gambling websites should have the same security feature of what exchanges do, it may be a matter of # of funds stored in its hot wallet but it is still a fund to run the business and should be considered in high level of security.
Cold wallet is just a back-up plan and it can’t really prevent hackers, better to improve the whole system and increase the security on every account to avoid any big losses in the future.
I believe on exchanges security are doing great but I think gambling site can so better than them since there’s also a big money in gambling, they should invest more on their security so gamblers will have a peace of mind trusting that specific site.